Solid-liquid separation is an important process in various industries. Water is often the liquid, as it is often the cheapest medium in which various industrial processes are carried out. The process of dewatering can be achieved by either mechanical methods (e.g., filtration and centrifugation) or thermal drying. In general, the former is cheaper than the latter. However, mechanical dewatering becomes increasingly difficult with decreasing particle size.
In a mechanical dewatering process, the particulate materials present in a feed slurry quickly form a bed (or cake) of particles on a medium before the water flows through the cake. The water flow rate, Q, through the cake is determined by the Darcy""s law:                     Q        =                              K            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            PA                                μ            ⁢                          xe2x80x83                        ⁢            L                                              [        1        ]            
where K is the permeability of the cake, xcex94P the pressure drop across the cake, A the filter area, xcexc the dynamic viscosity of water, and L is the cake thickness. The driving force for the flow of water is the pressure drop. In vacuum filtration, the pressure drop is less than 1 atm, while in pressure filtration pressures as high as 8-10 atm are applied. In centrifugal filtration, the pressure drop is created by centrifugal force.
A filter cake is considered to consist of capillaries of various radii, which are determined by the size distribution of the particles constituting the filter cake. In a given capillary of radius, r, the water will flow through the capillary if the pressure drop, xcex94P, exceeds the pressure of the water inside the capillary. The capillary pressure, p, is given by the Laplace equation:                               p          =                                    2              ⁢              γ              ⁢                              xe2x80x83                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              θ                        r                          ,                            [        2        ]            
where xcex3 is the surface tension of water and xcex8 is the contact angle of the particles in a filter cake. The contact angle is a measure of the hydrophobicity (water-hating property) of the particles.
Eq. [2] suggests three ways of achieving low cake moistures after filtration. These include i) surface tension lowering, ii) capillary radius enlargement, and iii) contact angle increase. Various chemicals (dewatering aids) are used to control these parameters. One group of reagents is the surfactants that can lower the surface tension. Most of the surface tension lowering agents used in industry are ionic surfactants with high hydrophile-lipophile balance (HLB) numbers, which tend to reduce contact angles and, hence, are detrimental to dewatering. Another group of reagents used as dewatering aids are inorganic electrolytes and organic polymers that are used as coagulants and flocculants, respectively. Both of these reagents are designed to increase the particles size and hence increase the capillary radius. However, they too tend to increase the contact angle of the particles, as they are hydrophilic in nature.
The U.S. Pat. No. 5,670,056 teaches a method of using non-ionic (or neutral) low HLB surfactants and water-soluble polymers as hydrophobizing agents that can increase the contact angle. Mono-unsaturated fatty esters, fatty esters and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. The fatty esters were used with or without using butanol as a carrier solvent. In a U.S. patent disclosure (Ser. No. 09/327,266), methods of using various other low HLB surfactants as dewatering aids are taught. In another U.S. patent application (Ser. No. 09/326,330), methods of using lipids are disclosed. The primary role of these reagents is to increase the contact angle of the particles to be dewatered. However, they also enlarge particles via hydrophobic coagulation and reduce surface tension. Thus, the dewatering aids disclosed in the pending applications addresses all of the three parameters, i.e., surface tension, contact angle, and capillary radius, toward the right direction. The net result of using such reagents as dewatering aids is that the rate of dewatering (given by Eq. [1]) is vastly higher than other dewatering aids, which gives rise to lower cake moistures.
An advantage of using the lipids as dewatering aids, as disclosed in U.S. patent application with a Ser. No. 09/326,330, is that they are of low cost and environmentally safe to use. Lipids are naturally occurring hydrophobic organic molecules isolated from biological cells and tissues. Animal fats and vegetable oils are the most widely occurring lipids, which are triesters of glycerol with three long-chain carboxylic acids. The performance of these reagents is slightly inferior to those of the low HLB: surfactants disclosed in the U.S. patent application with a Ser. No. 09/327,266, which may be attributed to the likelihood that the lipid molecules are too large to form close-packed monolayers of hydrophobes on the surface of the particles to be dewatered.
It is an object of the present invention to provide a novel method of decreasing the moisture of fine particulate materials during mechanical dewatering processes such as vacuum filtration, pressure filtration, and centrifugal filtration.
Another important object of the invention is to increase the rate at which water is removed so that given dewatering equipment can process higher tonnages of particulate materials.
Still another object of the instant invention is the provision of a novel dewatering method that creates no adverse effects on up- and downstream processes when the water removed from the dewatering processes disclosed in the present invention is recycled.
Yet another object of the invention is the provision of methods of controlling the frothing properties of the flotation product.
Perhaps the most important object of the instant invention is to achieve all of the above objects using low-cost affordable dewatering aids that have no harmful effects on the environment and the human health.
It is the most important object of this invention to provide an efficient method of dewatering fine particulate materials. This is achieved by destabilizing the water on the surface of the particles to be dewatered by rendering the surface substantially hydrophobic. The particles are hydrophobized normally in two steps. Initially, surfactants of high hydrophile-liphophile balance (HLB) numbers are used to render a particulate material moderately hydrophobic. The material is subsequently treated with a modified lipid to further enhance its hydrophobicity close to or above the water contact angle of 90xc2x0. This will greatly decrease the pressure of the water in the capillaries formed between the particles in a filter cake, and thereby allow the water to be removed more readily during mechanical dewatering processes.
A key to the methods of dewatering described in the present invention disclosure is the hydrophobicity enhancement step. According to the Laplace equation, a relatively small increment in hydrophobicity (above the level that can normally be achieved using a high HLB surfactant in the first hydrophobization step) can bring about a large decrease in the capillary pressure. The initial hydrophobization step may be omitted, if the particulate material is naturally hydrophobic or has been hydrophobized in an upstream process (e.g., flotation) preceding dewatering. However, the particles must remain reasonably hydrophobic at the time of the hydrophobicity enhancement step. Otherwise, the dewatering aids added in this step do not adsorb on the surface of the particulate material and fail to enhance its hydrophobicity.
In the present invention, naturally occurring lipids of vegetable and animal origin are broken into smaller molecules, so that they can more readily form close-packed layers of hydrophobes and, hence, greatly enhance the hydrophobicity of the particles. The lipid molecules are transesterified by reacting with alcohols in an appropriate catalyst to form monoesters, interesterified with glycerol to form mono- and diacylglycerols, and saponified and then acidulated to form fatty acids. The reaction products are used directly as dewatering aids without purification, which will keep the costs of the reagents acceptable for dewatering materials of relatively low value such as coal and mineral fines.
The modified lipids used in the second hydrophobization step of the present invention are insoluble in water; therefore, they are used as solutions in appropriate solvents, which include but not limited to light hydrocarbon oils and short-chain alcohols. The modified lipid molecules may act as low HLB surfactants that can greatly enhance the hydrophobicity of the particulate material to be dewatered.
The dewatering methods disclosed in the instant invention are capable of greatly increasing the dewatering rate and, hence, reduce the final cake moisture. Furthermore, the dewatering aids of the present invention have the characteristics of anti-forming agents, which is important for processing the particulate materials produced from flotation processes. Also, most of the reagents added as dewatering aids and blends thereof adsorb on the surfaces of minerals and coal fines so that the plant water does not contain significant amounts of residual reagents.
The difficulty in removing water from the surface of fine particles may be attributed to the fact that water molecules are held strongly to the surface via hydrogen bonding. One can break the bonds and remove the water by subjecting the wet particles to intense heat, high-pressure filters and high-G centrifuges. However, the use of such brute forces entails high energy costs and maintenance problems. A better solution would be to destabilize the surface water by appropriate chemical means, so that it can be more readily removed by the weaker forces imparted by vacuum or low-pressure filters.
The affinity of water adhering to the surface of a solid may be best represented by the hydrophobicity of the surface. The stronger the hydrophobicity, the lower the affinity. One may use appropriate reagents to increase the hydrophobicity and destabilize surface water. A traditional measure of surface hydrophobicity is water contact angle. In the cessile drop technique, contact angle is measured by placing a droplet of water on the surface of a solid of interest. The contact angle, measured through the aqueous phase, increases with increasing hydrophobicity.
In the present invention, particles in aqueous slurry are hydrophobized in two steps. In the first step, an appropriate surfactant is added to the slurry, so that it can adsorb on the surface of the particles to be dewatered and render them moderately hydrophobic. The contact angle of the particulate material may be increased to the range of 25 to 60xc2x0. For hydrophilic particles, high HLB surfactants are used to bring the contact angle to this range. In the second step, a modified lipid is added to the slurry to further increase the contact angle over 60xc2x0, preferably close to or over 90xc2x0. The hydrophobicity-enhancing step is essential for destabilizing the surface water and, thereby, expedites the process of mechanical dewatering. The first hydrophobization step may be omitted if the particles are moderately hydrophobic by nature or by virtue of an upstream process.
A pending U.S. patent application (Ser. No. 09/327,266) also discloses the advantages of incorporating a second hydrophobization step. In this application, well-defined low HLB surfactants are used as the hydrophobicity-enhancin reagent. However, many of the low HLB surfactants are considerably more expensive than the lipids disclosed in the present invention.
Another pending U.S. patent application (Ser. No. 09/326,330) discloses a method of incorporating a second hydrophobization step, in which the hydrophobicity-enhancing reagents are lipids. These are naturally occurring organic molecules that can be isolated from plant and animal cells (and tissues) by extraction with nonpolar organic solvents. Large parts of the molecules are hydrocarbons (or hydrophobes); therefore, they are insoluble in water but soluble in organic solvents such as ether, chloroform, benzene, or an alkane. Thus, the definition of lipids is based on the physical property (i.e., hydrophobicity and solubility) rather than by structure or chemical composition. Lipids include a wide variety of molecules of different structures, i.e., triacylglycerols, steroids, waxes, phospholipids, sphingolipids, terpenes, and carboxylic acids. They can be found in various vegetable oils (e.g., soybean oil, peanut oil, olive oil, linseed oil, sesame oil), fish oils, butter, and animal oils (e.g., lard and tallow). Although fats and oils appear different, that is, the former are solids and the latter are liquids at room temperature, their structures are closely related. Chemically, both are triacylglycerols; that is, triesters of glycerol with three long-chain carboxylic acids. They can be readily hydrolyzed to fatty acids. Corn oil, for example, can be hydrolyzed to obtain mixtures of fatty acids, which consists of 35% oleic acid, 45% linoleic acid and 10% palmitic acid. The hydrolysis products of olive oil, on the other hand, consist of 80% oleic acid. Waxes can also be hydrolyzed, while steroids cannot. Vegetable fats and oils are usually produced by expression and solvent extraction or a combination of the two. Pentane is widely used as a solvent, and is capable of extracting 98% of soybean oil. Some of the impurities present in crude oil, such as free fatty acids and phospholipids, are removed from crude vegetable oils by alkali refining and precipitation. Animal oils are produced usually by rendering fats.
In the present invention, hydrophobic lipids are modified so that they can more readily form closed-packed hydrophobes on the surface of the particles to be dewatered. The triacylglycerols present in the naturally occurring lipids may be considered to be large surfactant molecules with three hydrocarbon tails, which may be too large to form close-packed monolayers of hydrophobes. A solution to this problem is to break the molecules into smaller ones before using them as dewatering aids. In one method, triacylglycerols are subjected to transesterification reactions to produce monoesters. Typically, an animal fat or oil is mixed with an alcohol and agitated in the presence of a catalyst usually H+ or OHxe2x88x92 ions. If methanol is used, for example, methyl fatty esters of different chain lengths and structures are formed along with glycerol. The reactions can be carried out at room temperature; however, the reactions may be carried out at elevated temperature in the range of 40 to 80xc2x0 C. to expedite the reaction rate.
In another method, triacylglycerols are hydrolyzed to form fatty acids. They can be hydrolyzed in the presence of H+ or OHxe2x88x92 ions. In the case of using the OHxe2x88x92 ions as catalyst, the fatty acid soaps formed by the saponification reactions are converted to fatty acids by adding an appropriate acid.
In still another method, triacylglycerols are reacted with glycerol to produce a mixture of esters containing one or two acyl groups. This reaction is referred to as interesterification.
The process of breaking the lipid molecules are simple and, hence, do not incur high costs. Furthermore, the reaction products may be used without further purification, which contributes further to reducing the reagent costs.
The acyl groups of the naturally occurring lipids contain even number of hydrocarbons between 12 and 20, and may be either saturated or unsaturated. The unsaturated acyl groups usually have cis geometry, which is not conducive to forming close packed monolayers of hydrocarbons. Some of the lipids have higher degrees of unsaturation than others. Therefore, it is desirable to either use the lipids of lower degree of unsaturation, or use the lipids of high degree of unsaturation after hydrogenation. The hydrogenation can decrease the degree of unsaturation of the acyl groups. This technique can be applied before or after breaking the triacylglycerols to smaller molecules using the methods described above.
Since the modified lipids have: low HLB numbers, they may be used as solutions of appropriate solvents including but not limited to short-chain alcohols and light hydrocarbon oils. Typically, one part by volume of a lipid, which may be termed as active ingredient(s), is dissolved in two parts of a solvent before use.
The high HLB surfactants used in the first hydrophobization step adsorb only on specific surface sites. The population of the surface sites, at which the adsorption can occur, is usually well below what is needed to form a close-packed monolayer of the adsorbed surfactant molecules. The modified hydrophobic lipids that are used in the second hydrophobization step may adsorb in between the sparsely populated hydrocarbon tails of the high HLB surfactants, so that the surface is more fully covered by a close-packed monolayer of hydrophobes. It has been shown that the hydrocarbon tails of the surfactant molecules adsorbing on the surface of a solid begin to stand up vertically and form a close-packed monolayer at a contact angle close to or above 90xc2x0 (Flinn, et al. Colloids and Surfaces A, vol. 87, p. 163, 1994). It has also been shown that the force of attraction between two hydrophobic surfaces increases sharply at contact angle 90xc2x0 (Yoon and Ravishankar, J. Colloid and Interface Science, vol. 179, p. 391, 1996). That the modified lipids used in the present invention are smaller in molecular size than the naturally occurring-lipids should be conducive to forming close-packed monolayers of hydrophobes and, hence, increasing contact angles. Also, the use of lipids whose acyl groups have a higher degree of saturation should be more conducive to forming closed-packed monolayers of hydrophobes.
Several different coal samples were used for a series of laboratory-scale dewatering tests. In a given test, a volume of coal slurry was placed in an Erlenmeyer flask, into which a known amount of reagent(s) was added and agitated for 2 minutes. The conditioned slurry was poured into a 2.5-inch Buchner funnel with glass frit of medium porosity, which in turn was mounted on a vacuum chamber. After a preset drying cycle time (usually 2 minutes), the product was removed from the Buchner funnel and analyzed for moisture.